Fuel cell assemblies convert a fuel and an oxidant to electricity. One type of fuel cell power system employs a proton exchange membrane (hereinafter “PEM”) to separate electrodes that facilitate catalytic reaction of fuels (such as hydrogen) and oxidants (such as air or oxygen) to generate electricity. The PEM is typically a solid polymer electrolyte membrane that facilitates transfer of protons from an anode to a cathode in each individual fuel cell normally deployed in a fuel cell power system.
In a typical fuel cell assembly (or stack) within a fuel cell power system, individual fuel cell plates include channels through which various reactants and cooling fluids flow. Fuel cell plates are typically designed with straight or serpentine flow channels. Such flow channels are desirable as they effectively distribute reactants over an active area of an operating fuel cell, thereby maximizing performance and stability. In subzero temperatures, water vapor in the fuel cell assembly may condense. The condensate may form ice in the fuel cell assembly. The presence of condensate and ice may affect the performance of the fuel cell assembly.
During typical operating conditions, condensate may also accumulate at the edges of the fuel cell plates adjacent outlet manifolds of the fuel cell assembly, thereby restricting fluid flow from the flow channels to the outlet manifolds. During a starting operation of the fuel cell assembly in subzero temperatures, the condensed water in the flow channels of the fuel cell plates and at the edges of the outlet manifolds is in the form of ice which may restrict reactant flow. Similarly, reactant flow maldistribution due to liquid water stagnation during normal operation can result. Furthermore, thermal energy generated by the fuel cells of the assembly is lost to end units of the assembly, thereby delaying the heating of the assembly.
Typically, to mitigate the formation of condensation at the outlet manifolds of the fuel cell assembly, the operating temperature of the fuel cell assembly is increased. However, increasing the operating temperature may have a negative impact on ohmic resistance due to increased membrane proton resistance as a result of decreased membrane humidification. Also, decreasing the relative humidity of inlet anode and cathode gas streams may achieve the same effect as increasing the operating temperature and result in a negative impact on ohmic resistance due to increased membrane proton resistance. To mitigate thermal losses to the end units of the assembly, a thermally resistive barrier layer may be disposed between the fuel cell stack and the end units. As thermal resistivity increases, electrical conductance decreases, thereby generating waste heat in the barrier layer and causing the fuel cell assembly to operate inefficiently. To withstand the elevated temperatures that can arise at high current levels due to this waste heat generation in the barrier layer, the end units must be formed from expensive plastics or other materials able to withstand elevated temperatures, thereby increasing the cost of the fuel cell assembly.
During operation of the fuel cell assembly, waste heat from the fuel cell reaction heats the fuel cell assembly and mitigates water condensation and ice formation in the assembly. However, end plates of the fuel cell assembly tend to have a temperature lower than the temperature of intermediate plates of the fuel cell assembly. The end plates have a lower temperature due to thermal losses to the environment and thermal losses to terminal plates and end units of the fuel cell assembly adjacent thereto. A difference in the temperature of the fuel cell plates throughout the fuel cell assembly may result in inefficient operation, maldistribution of reactants, condensation of water on the fuel cell plates which may lead to ice formation, and a decreased useful life of the fuel cell assembly.
Typically, to ensure a substantially uniform temperature distribution between the plates in the fuel cell assembly, a heating mechanism is disposed adjacent the end plates to directly transfer thermal energy thereto. A heating mechanism may also be disposed adjacent the terminal plates to transfer thermal energy thereto. Thermal energy is then transferred from the terminal plates to the end plates. Alternatively, a resistive heating mechanism adapted to heat the end plates may be connected in parallel to the fuel cell assembly. If a heating mechanism fails and is in a powered state, the end fuel cells may dry out, thereby leading to an electrical short in the fuel cell assembly. Other methods for heating the end plates include catalytic heating, and providing a bypass plate disposed between the end plates and the terminal plates.
Also, during operation of the fuel cell assembly, electrical current generated by the fuel cell stack is collected in each electrically conductive fuel cell. The current is transmitted through the stacks, via the fuel cell plates, to terminal plates at either end of the fuel cell stack. The terminal plates are in electrical communication with a current collecting body, such as a bus bar, for example. The current collecting body is in electrical communication with a stack interface unit (SIU) or other electrical components of the fuel cell power system. High temperatures in the stack end units will cause heat to flow with the electrical current to the SIU and/or other electrical components, thereby resulting in increased temperatures in the SIU and/or other electrical components which may result in component failure or requiring costly components that can operate at elevated temperatures.
It would be desirable to develop a fuel cell assembly having a barrier layer disposed between a terminal plate and an end plate thereof, the barrier layer having variable properties to facilitate startup thereof in freezing external temperatures and to minimize the temperature in an end unit of the assembly at high current levels.